NAO Data Hunting (and Gathering)

While converting Greenlanders in the 1770s, Danish Christian missionary Hans Egede Saabye noticed something. In his diary, he penned: “In Greenland, all winters are severe, yet they are not alike…when the winter in Denmark was severe, as we perceive it, the winter in Greenland in its manner was mild, and conversely.”

Sounds like the North Atlantic Oscillation to us. But little did Saabye know that beyond local winter temperature twists, the NAO flexes serious climatic and ecological muscle over much of the Northern Hemisphere. When the NAO perturbs wintertime temperatures on land and sea, storm activity, precipitation, or ocean currents, snowball effects can ensue: Ecosystems can get knocked off-kilter. Marine-life food supplies can fluctuate wildly. Populations of land mammals can sink or soar.

Uncovering the NAO’s true chain-reaction impacts hinges on one factor--high-quality scientific data taken over long periods of time. And the climatic and biological data that have been hoarded for decades are, as of late, finally hitting NAO pay dirt.

Trawling for Information

“It’s very important to collect data over time for climate,” says NOAA marine forecaster Scott Prosise. “That way, we can establish a baseline against which everything can be measured.” Prosise talks shop while on deck of the Oleander, a commercial container ship. As it does every week, the 386-foot-long vessel is chugging the 650 miles from Port Newark, NJ to Bermuda, shuttling groceries, clothing, and other consumer goods to the island. Prosise is one of many scientific volunteers invited aboard by the ship’s owners, the Bermuda Container Line, to log sea surface temperatures, wind speed, salinity, current velocities, plankton levels, and other data en route in the North Atlantic.

Such information, collected for decades by merchant vessels and Navy and Coast Guard fleets, have formed the backbone of marine climate datasets. These collections of numbers over time have enabled researchers to flesh out how the NAO is influencing the oceans and vice versa.

Scott Prosise aboard the Oleander

Prosise aims what appears to be a large water pistol toward the waves. He tugs on the launcher’s holding pin, releasing a 4-inch metal probe of an expendable bathythermograph into the water. As it sinks to the seafloor, the bathythermograph continuously measures changing sea temperatures at successive depths, transmitting the data via a wire connected to a computer on deck.

This constantly moving marine layer cake of cold and warm depths has been found to change dramatically under NAO influence. Such changes have notable repercussions, like the increased frequency of US-east-coast-bound hurricanes during a positive year (fueled by warmer currents between Bermuda and Florida) and the devastation of Canadian cod stocks (thanks to a colder Labrador Sea).

Atlantic datasets have even proven that the NAO has the power to bully the Gulf Stream itself: Since positive phases shift the westerly storm track northward, the winds help nudge the major Atlantic current accordingly, about 100 km to the north. Such shifts string along Atlantic tuna fishermen, who seek good catches at the northern edge of the warm Gulf Stream.

Still, single boats like the Oleander crisscrossing the wide ocean freeway can’t provide the breadth of monitoring needed for a nationwide marine dataset of the highest caliber. The prototype? A team of 10 “smart” buoys bobbing innocuously about the Gulf of Maine. Called GoMOOS, the 3-year-old Gulf of Maine Ocean Observing System is setting a new standard for incredibly thorough, 24/7, freely available measurements of currents, temperature, salinity, dissolved oxygen, phytoplankton biomass, and more.

Researchers have determined that a positive NAO channels warm, salty water into the Gulf of Maine (spurred, actually, by the northerly shift of the warm Gulf Stream). In negative NAO years, cold, fresher water floods the Gulf. These two conditions can alternately affect local ecosystems. As GoMOOS’ datasets accumulate over decades, they’ll enable science teams to more clearly understand the NAO’s impacts on the Gulf, including plankton abundance (and its effect on the 300 surviving North Atlantic right whales that eat the plankton), red tides and other algal blooms, lobster reproduction, and other phenomena.

Wish You Were Here

How long a dataset do NAO researchers need, really--years, decades, or hundreds of years? Though project-dependent, generally the longer the time frame of study, the more clearly patterns will emerge. And for those striving to put the NAO’s current variability in a context of its historical, “natural” index, the best dataset is the longest one you can find.

Easier said than done. People simply weren’t sticking barometers in Iceland and the Portugal area and reading them regularly before about 1821. In the absence of sea level pressure data, which directly calculates the index, scientists must take poetic license--they extrapolate winter indices for previous centuries of the NAO from other available information. Called proxies, such data “libraries” are found in natural objects that bear climate imprints from times past.

Dendroclimatologist Ed Cook

One researcher — dendroclimatologist Ed Cook — uses tree rings as his proxy. Together with other proxies like Greenland ice cores, he’s managed to reconstruct a virtual winter NAO index to the year 1400. Cook , who calls himself “Dr. Dendro” and looks suspiciously Paul-Bunyan-esque, is the director of Columbia University’s Tree Ring Lab at their Lamont-Doherty Earth Observatory. He has traveled to key North American and European forests, coring thousands of geriatric trunks with an arborist’s version of a hypodermic needle. Cook and his colleagues then analyze the cores’ ring widths for clues about previous rainfall and temperature patterns in the area. A narrow ring reflects a year in which climate was poor for growth. A wide ring records a year of favorable climate conditions.

Since each ring represents a single growth year, Cook can peg long-gone climates with eerie accuracy. “We can say, for example, what the climate was like for the Battle of Hastings in the year 1066, simply from ring growth contained in British oaks that grew back then,” he marvels.

So how do you get a 600-year record from a 300-year old tree? “We find pieces of remnant wood from the same species nearby,” says Cook. By matching a telltale ring series for several years in both the live and dead tree, dendroclimatologists can overlap the patterns to create an extended paleoclimate historyback 10,000 years for some areas of the world!

The reconstructed NAO index from proxies is still not complete to make a definitive claim about the NAO’s natural variability patterns for the long haul. But according to Cook, the indication so far is that the positive trend we’ve been experiencing since the early 70s is rare, but not necessarily unheard of, within the past 600 years. “That makes it harder to argue that the NAO is being uniquely forced into a brand new mode of variability that has never been seen before,” he says. That said, as is often the case for many arguments about global warming, the jury’s still out.